The present invention relates to a semiconductor device including a group III nitride semiconductor represented by a general formula, InxAlyGa1−x−yN (wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1), and more particularly, it relates to a semiconductor device including an oxide film formed by oxidizing a group III nitride semiconductor and a method of fabricating the same.
A group III nitride semiconductor having a composition of InxAlyGa1-x-yN, that is, the so-called gallium nitride-based (GaN-based) compound semiconductor, is regarded as a promising material for a light emitting device such as an LED and a semiconductor laser diode because the interband transition of electrons is direct transition therein and its band gap is varied in a wide range between 1.95 eV and 6 eV.
Recently, particularly in order to realize higher density and higher integration of information processing equipment, semiconductor laser diodes capable of outputting light of a wavelength in a blue-violet region are earnestly developed. Also, since GaN has high breakdown field, high thermal conductivity and a high electron saturation velocity, it is a promising material also for a high frequency power device. In particular, a heterojunction structure including aluminum gallium nitride (AlGaN) and gallium nitride (GaN) has an electron velocity twice as large as that of gallium arsenide (GaAs) at high electric field as high as 1×105 V/cm to realize down sizing, and hence is expected a high frequency operation of a device.
Since a group III nitride semiconductor exhibits an n-type characteristic when doped with an n-type dopant including a group IV element such as silicon (Si) and germanium (Ge), application to a field effect transistor (FET) is now under development. Also, since a group III nitride semiconductor exhibits a p-type characteristic when doped with a p-type dopant including a group III element such as magnesium (Mg), barium (Ba) and calcium (Ca), application to an LED and a semiconductor laser diode including a pn-junction structure of a p-type semiconductor and an n-type semiconductor is now under development. As an applicable electronic device, a high electron mobility transistor (HEMT) including a heterojunction of, for example, AlGaN and GaN is widely being examined to be realized by using a group III nitride semiconductor having a high electron transporting property.
Now, a conventional AlGaN/GaN-based HEMT will be described with reference to drawings.
FIGS. 23A and 23B show the conventional AlGaN/GaN-based HEMT, wherein FIG. 23A shows the plane structure thereof and FIG. 23B shows the cross-sectional structure thereof taken on line XXIIIB-XXIIIB of FIG. 23A. As is shown in FIGS. 23A and 23B, a first HEMT 100A and a second HEMT 100B are formed on a substrate 101 of silicon carbide (SiC) so as to be separated by a scribe region 110, used for dividing the substrate 101 into chips each including a transistor.
Each of the first HEMT 100A and the second HEMT 100B includes, on a buffer layer 102 of GaN grown on the substrate 101, an active region 103 formed by mesa-etching a heterojunction layer of AlGaN/GaN.
On each active region 103, a gate electrode 104 in Schottky contact with the active region 103 and ohmic electrodes 105, in ohmic contact with the active region 103, disposed with space from side edges along the gate length direction of the gate electrode 104 are formed.
A portion above and around each active region 103 including the gate electrode 104 and the ohmic electrodes 105 is entirely covered with an insulating film 106, and pad electrodes 107 respectively electrically connected to the gate electrode 104 and the ohmic electrodes 105 are formed on each insulating film 106. The insulating film 106 is covered with a surface passivation film 108 with the pad electrodes 107 exposed.
The insulating film 106 covering the active region 103 is generally formed from silicon oxide or the like, so as to protect the surface of the active region 103 and ease formation of the gate electrode 104 by a lift off method.
As is shown in FIG. 23A, since it is necessary to provide the gate electrode 104 with an extended portion 104a to be connected to the pad electrode 107, the gate electrode 104 is formed not only on the active region 103 but also on the buffer layer 102 of GaN exposed by the mesa-etching.
In the conventional AlGaN/GaN-based HEMT, however, contact between the extended portion 104a and the buffer layer 102 is contact between a metal and a semiconductor, namely, the so-called Schottky contact, and hence, there is a problem that a leakage current tends to occur due to damage of the semiconductor surface caused in the mesa-etching. This leakage current largely affects a pinch-off characteristic of the transistor, resulting in degrading the transistor characteristic.
Furthermore, since adhesion between the buffer layer 102 of GaN and the insulating film 106 of silicon oxide is insufficient, there is another problem that the insulating film 106 peels off in wire-bonding the pad electrodes 107 formed on the insulating film 106.
Moreover, both the substrate 101 of SiC and the GaN-based semiconductor have high hardness, and hence, it is very difficult to conduct a scribe process for dividing the substrate into chips as compared with the case where Si and GaAs are used. Therefore, the yield may be lowered due to occurrence of a crack reaching the active region 103 in the scribe process or the reliability may be lowered due to peeling of the surface passivation film 108 or the insulating film 106 in the vicinity of the scribe region 110.
In a semiconductor laser diode having a laser structure formed by multi layers of group III nitride semiconductors, a substrate of sapphire is generally used. In the case where sapphire is used as the substrate, it is difficult to form a cavity structure by cleavage because of a difference in the crystal axis between sapphire and the laser structure formed on the sapphire, and hence, the cavity structure is frequently formed by dry etching. When the cavity is formed by dry etching, however, a defect peculiar to the formed cavity facet is caused so as to form a non-luminescent center. As a result, there arises a problem that the operation current (threshold current) may increase or the reliability may be lowered.